1. Field of the Invention
The present invention relates to a light emitting device and a manufacturing method thereof using a light emitting element which has a film containing an organic compound (hereinafter referred to as an “organic compound layer”) between a pair of electrodes and which can give fluorescence or luminescence by receiving an electric field. The light emitting device referred to in the present specification is an image display device, a light emitting device or a light source. Additionally, the following are included in examples of the light emitting device: a module wherein a connector, for example, a flexible printed circuit (FPC) or a tape automated bonding (TAB) tape, or a tape carrier package (TCP) is set up onto a light emitting element; a module wherein a printed wiring board is set to the tip of a TAB tape or a TCP; and a module wherein integrated circuits (IC) are directly mounted on a light emitting element in a chip on glass (COG) manner.
2. Description of the Related Art
A light emitting element in the present invention is an element for emitting light by applying an electric field thereto. With respect to the light emitting mechanism, it is said that an electron injected from a cathode and a hole injected from an anode are recombined in an organic compound layer by applying a voltage to electrodes sandwiching an organic compound layer to produce a molecule with an excitation state (hereinafter referred to as “a molecular exciton”) and the molecular exciton releases energy to emit light when it is returned to a ground state.
In such a light emitting element, the organic compound layer is generally made from a thin film having a thickness less than 1 μm. In addition, since the light emitting element is a self-luminous type element such that the organic compound layer itself emits light, a back light used in a conventional liquid crystal display is not required. Thus, it is the big advantage that an extremely thin and lightweight light emitting element can be manufactured.
Also, when the carrier mobility of, for example, an organic compound layer having a thickness of about 100 nm to 200 nm is considered, a period from the injection of a carrier to the recombination is about several ten nanoseconds. Even when a period required for a process from the recombination of a carrier to light emission is included in the period, light emission is conducted within the order of microsecond. Thus, an extremely high response speed is one of characteristics thereof.
From characteristics such as a thin type, lightweight, high speed responsibility, and direct-current low-voltage drive, the light emitting element has been noted as a next generation flat panel display element. In addition, since the light emitting element is a self-luminous type and has a wide viewing angle, the visibility is relatively good. Thus, it is considered that the light emitting element is effective as an element used for a display screen of a mobile electronic apparatus.
Tang et al. of Eastman Kodak company succeeded to obtain high luminance and high efficiency that are sufficient to put into practical application with luminance of 1000 cd/m2 at 10 V or less and external quantum efficiency of 1% by laminating organic compounds having different carrier transportation in order to improve the characteristics of elements in which holes and electros are injected from each anodes and cathodes with sufficient balance and the thickness is set to 200 nm or less (Reference 1: Appl. Phys. Lett., 51, 913 (1987)). With respect to the high quantum efficiency elements, Tang uses Mg (magnesium) having small work function to the organic compound that is basically regarded as insulators in order to lower the energy barrier generated with injection of electrons from metal electrodes. However, Mg is easily oxidizable, instable, and has poor adhesion property to the organic surface, so that Mg is co-deposited with Ag (argentine) that is relatively stable and has high adhesion property to the organic surface to be alloyed is used.
The group of Toppan Printing Company reported that lower driving voltage and higher luminescence than the element using Mg alloy by alloying Li (lithium) having smaller work function than Mg with Al (aluminum) in order to stabilize as to use as cathode (Reference 2: 51st Japanese Society of Applied Physics Annual Meeting, Digest 28a-PB-4, p. 1040).
In light of the background art of the above-mentioned alloy electrodes, it has been desired to develop more stable cathodes. In recent years, it has been reported that by interposing a cathode buffer layer made of lithium fluoride (LiF) or the like as a super-thin insulating layer (0.5 nm), even an aluminum cathode can give luminescence property equivalent to or more than that of alloy of Mg and Ag, or the like alloy (Reference 3: L. S. Hung, C. W. Tang and M. G. Mason: Applied Physics Letters, 70 (2), 152 (1997).
The mechanism of the property improvement by disposing this cathode buffer layer would be as follows: when LiF constituting the cathode buffer layer is formed to contact Alq3 constituting an electron transport layer of an organic compound layer, the energy band of Alq3 is bent to lower an electron injection barrier.
Another report is that improving the injection of electron from cathode by reducing the electron injection barrier from cathode to the organic compound layer by forming a metal doping layer in the organic compound layer contacting with a cathode of the light emitting element, the doping layer is made from one or more metals from the following: alkali metals having 4.2 eV or more work functions, alkaline earth metals, and transition metals including rare earth metals (Japanese Patent Application laid-open No. Hei 10-270171).
As described above, in a light emitting element composed of an anode, a cathode and an organic compound layer, an invention is made for improving the capability of injecting carriers from the electrode, resulting from an element characteristic of the light emitting element.
Further, there is another report that the carrier density according to the improvement of hole injection and conductivity can be increased by doping the electron acceptance materials to the hole transporting layer contacting with the anode of the light emitting element. Thus, a low driving voltage can be realized (Reference 4: J. Blochwitz, M. Pfeiffer, T. Fritz, and K. Leo, Applied Physics Letters, vol. 73, No. 6, p. 729 (1998)).
However, with respect to the active matrix light emitting device, when above-described cathode buffer layer and metal doping layer are provided between the organic compound layer and the cathode in order to improve the element characteristics of the light emitting element, the injection from the cathode is improved, meanwhile, TFT characteristics is deteriorated since the alkali metals and alkaline earth metals contained in a part of the cathode buffer layer and the metal doping layer are diffused to drift, and the TFT connected to the light emitting element is influenced by that. Thus, the characteristics of the light emitting element is improved, but then, the characteristics of TFT is deteriorated.
By doping the electron acceptance material, the carrier density according to the improvement of the hole injection and conductivity can be increased, meanwhile, the organic compounds functioning as an acceptor is possible to form a charge-transfer complex. In the case that the charge-transfer complex exists at the interface with the light emitting layer, an energy produced by recombination of carrier generated on the light emitting layer moves to a non-luminous charge-transfer complex to be quenched. The same is equally true of donor that improves electron injection by doping the electron dose materials.